To increase the combustibility and the ignition ability of an internal combustion engine, it is necessary to remove from the fuel oil heavy, oxidized deteriorating oily products deposits comprising high molecular hydrocarbons which contain impurities, such as asphaltene; such deteriorating products will be referred to hereinafter as sludge.
Conventionally the fuel oil for use, for example, in a marine engine is subjected to one or more of the following methods on the ship so that the sludge is removed.
(a) A method wherein a sedimentation tank is used.
(b) A method wherein a centrifugal separator is used.
(c) A method wherein a filter having a mesh size of between 45 .mu.m and 75 .mu.m is used.
It is well known that, as the grade of the fuel oil is lowered, the sludge content in the fuel oil is considerably increased and its specific gravity is also increased. However, there is not an obvious difference in specific gravity between the sludge and the degraded fuel oil, because the sludge essentially comprises hydrocarbons and comparatively small amounts of ash content. It is therefore difficult to remove the sludge contained in the degraded fuel oil in accordance with the above-mentioned methods (a) or (b) wherein the difference in the specific gravity is utilized. When the method (c) is carried out for removing sludge contained in the degraded fuel oil, the mesh size applied in a conventional filter is too large. More specifically, the mesh size of a filter utilized in the above-mentioned conventional method (c) is larger than 45 .mu.m, and, as a result, only particles larger than 45 .mu.m in diameter are strained out by means of the filter. It has been said, however, that it is preferable for increasing the combustibility of an internal combustion engine to strain out sludge contained in fuel oil, the particle size of which sludge is between 5 .mu.m and 30 .mu.m. As is apparent from the above description, the above-mentioned method (c) is not satisfactory for increasing the combustibility of an internal combustion engine. To strain out sludge, the particle size of which is between 5 .mu.m and 30 .mu.m, it is necessary to use a fine mesh filter for fuel oil utilized in a marine engine. In such a case, a sufficient filtering capacity cannot be obtained even though the pressure difference exerted on the filter is increased, because the filtrate has a large surface tension whereby making the rate of filtration very slow through such a small mesh filter and also the filter may become clogged in a short period. As a result, it is necessary to have a large filtering surface, and therefore, the conventional apparatus wherein the method (c) is carried out is large in size and high in cost.
It will be understood from the description above that it becomes more difficult to remove the sludge when the above-mentioned methods (a) through (c) are carried out, as the fuel oil is degraded and as the sludge content is increased. As a result, the sludge, which is difficult to burn in a diesel combustion chamber as it is, causes incomplete combustion, and carbon adheres to the injection nozzles of the diesel engine as carbon flower when fuel oil with sludge suspended in it is fed to the engine. The carbon flower prevents the atomization of the fuel oil, and in addition, when carbon adheres between a valve and the corresponding valve seat or between a piston ring and the corresponding cylinder liner, it may be become a main cause of wearing and acid corrosion of the valve, valve seat, ring and liner. The sludge which is contained in fuel oil results in instable performance of the diesel engine when in operation, particularly in middle and high speed diesel engines.
By a conventionally known filtering system wherein ultrasonic means are utilized fuel oil can be somewhat reformed, however, the efficiency for irradiating ultrasonic waves is not high because, in the conventional system, ultrasonic waves are irradiated within the whole container wherein fuel oil is stored. As a result, the strainer utilized in the conventional method is clogged by sludge in a very short time, and it is necessary to discontinue reforming the fuel oil. In addition, the conventional method wherein ultrasonic waves are utilized has a disadvantage in that the amplitude is too small, and therefore, cleaning of the strainer cannot be effected sufficiently. Based on the tests conducted by the present inventors, it was confirmed that the cleaning operation of the filter medium was achieved by exerting suction on the particles caked on the filter by utilizing a vacuum pressure generated within a gap which is smaller than several millimeters and which is formed between the end of a tapered horn and the surface of the layered mesh structure. To realize the suction effect due to the vacuum pressure, an amplitude in a range higher than 20 .mu.m from peak to peak is required. However, in the conventional method wherein ultrasonic waves are utilized, the amplitude was extraordinarily small, i.e., about several .mu.m from peak to peak. Accordingly, except for the particular case wherein the condensation of sludge which cause clogging of the filter medium is low, the conventionally known method wherein liquid is continuously filtered while the filter medium is being cleaned by utilizing ultrasonic waves has been unsatisfactory. However, the condensation of the sludge contained in the fuel oil which is to be treated by means of an apparatus of the present invention is too high to remove in accordance with the conventionally known method wherein the fuel oil is filtered while ultrasonic waves are utilized.